Vascular endothelial growth factor (VEGF) is critical for tumor angiogenesis and tumor immune suppression. Its elevated expression in tumors has been correlated with increased metastasis, resistance to conventional therapeutic agents and poor survival rates for cancer patients. However, effective anti-VEGF therapy remains a challenge in part because numerous transforming events, including activation of oncogenic kinases and inactivation of tumor suppressor genes, induce VEGF expression. Recently, we have identified Stat3 as a novel transcriptional regulator of the VEGF promoter. Significantly, Stat3 is constitutively-activated with 50-90% frequency in diverse cancers and many known VEGF-inducers signal through Stat3. Moreover, the tumor suppressors, PTEN and p53, are important negative regulators of VEGF gene expression. PTEN has recently been shown to inhibit Stat3 activity, suggesting that loss of PTEN activity would increase Stat3-mediated VEGF up-regulation. Furthermore, our preliminary studies have shown that Stat3 inhibits p53 expression, indicating a role of Stat3 in negating p53 effects on VEGF down-regulation. In addition, our preliminary results show that Stat3 signaling is required for induction of hypoxia-inducing factor 1 (HIF-1) alpha. Prior to discovery the Stat3/VEGF connection, HIF-1 was the only known VEGF transcriptional factor and its important role in mediating VEGF induction by oncogenic kinases and the p53/PTEN tumor suppressors has been established. These findings led us to hypothesize that Stat3 is not only a direct regulator of the VEGF promoter but also an effector and/or regulator of numerous other VEGF inducers. Our results demonstrate that binding of Stat3 to the VEGF promoter is obligatory for Src tyrosine kinase-induced VEGF up-regulation. Here we propose to determine whether Stat3 also transmits signals from other VEGF inducers up-stream of Stat3 directly to the VEGF promoter. We will also assess the molecular mechanism(s) by which Stat3 controls HIF-1 alpha expression. Finally, because Stat3 blockade inhibits VEGF expression and angiogenesis while at the same time down-regulates several key antiapoptotic genes and induces tumor cell apoptosis and tumor regression, we will have a unique opportunity to assess how endothelial cell death/vessel collapse affects tumor cell apoptosis/tumor regression and vice versa. Collectively, these studies may lead to development of more potent VEGF inhibitors for cancer therapy. [unreadable] [unreadable]